In most prior art fuel assemblies, the plates are provided with springs situated opposite bosses and exerting a force on the rods to hold them against the bosses. However, changes in reactor design particularly towards higher burn-up and higher thermohydraulic performance, have led to changes in the functions of the various grids. In particular, it is presently considered advantageous to use a single grid to support the fuel rods, with the other grids belonging to two possible different types. One of these types has a structural role and includes a belt, while the other type has a role that is purely thermohydraulic.
Proposals have already been made to provide grids (other than the grids which support the fuel rods) with bosses placed at one or more levels in the coolant flow direction, without any springs. To avoid damage to the rod sheaths during insertion, the gaps between the bosses leave clearance for the rods. However, manufacturing tolerances are such that leaving residual clearance for the largest diameter rods imply that the clearance left for the smallest diameter rods and the largest cells can reach a high value of about 0.4 mm. This clearance may increase further while the assembly is under irradiation due to creeping of the rod sheath or expansion of the cell, and this can lead to sufficient clearance to enable vibrations to reach amplitudes that will damage sheaths.